OP_CTV support for elements

Copied from https://github.com/ElementsProject/elements/pull/949#issuecomment-870125117

Challenges:

- Directly migrating OP_CTV is tricky because fees in elements are output UTXO. The value cannot be known statically while receiving money in CTV controlled output. We would have to modify the template w.r.t to not include fee outputs.
- It might require more thinking about how the templating would interact with confidential assets. Just directly using the same template from bitcoin BIP 119 might not be the best way.
- All the functionality which OP_CTV allows is permitted by the current design. You can directly fix the transaction structure by looking at the outputs themselves. The solution is more generic and flexible than a fixed template. You can emulate any specific template that you want here.
- The best selling point about BIP119 is IMO the tooling around it(Sapio). But I don't think we have the bandwidth to extend/maintain Sapio for elements.

Keine Quelldatei, kein Test und kein Einstiegspunkt ist angegeben. Beginne mit der Durchsicht der verlinkten BIP 119-Diskussion und der aufgeführten Herausforderungen im Zusammenhang mit Gebühren und vertraulichen Assets; die Abschlusskriterien für die Unterstützung von OP_CTV in Elements müssen noch definiert werden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.